Individual items in visual working memory inherit ensemble properties

Abstract

Prevailing theories of visual working memory assume that each encoded item is stored or forgotten as a separate unit independent from other items. Here, we show that items are not independent, and that the recalled orientation of an individual item is strongly influenced by the summary statistical representation of all items (ensemble representation). In Experiment 1, we briefly shown our participants a set of four triangles of various orientations and asked them to remember an orientation of (1) a single triangle, (2) all four individual triangles, or (3) the mean orientation of the four triangles. The orientations were distributed with equal steps along a range of 30, 60, and 120 degrees. When observers remembered one orientation, their memories had always the same precision regardless of range and were mostly unbiased. However, when observers had to remember all four orientations, not only was memory for an individual orientation substantially biased towards the mean orientation, but the precision of memory for an individual item also closely tracked the precision with which people stored the mean orientation (which was, in turn, correlated with the physical range of orientations). Thus, individual items are reported more precisely when items on a trial are more similar. Moreover, the narrower the range of orientations present on a trial, the more participants appear to rely on the mean orientation as representative of all individuals. This pattern was replicated when we discouraged observers from explicit encoding of the mean (Experiment 2). This ensemble influence can also be observed not only when the range is carefully controlled, but also shown even in randomly generated, unstructured displays (Experiment 3). Our results suggest that the information about a set of items is represented hierarchically, and that ensemble information can be an important source of information to constrain uncertain information about individuals.